EXTRAVASCULAR DIFFUSION OF TIRAPAZAMINE - EFFECT OF METABOLIC CONSUMPTION ASSESSED USING THE MULTICELLULAR LAYER MODEL

Purpose: Hypoxia-selective cytotoxic agents, like tirapazamine (TPZ), must diffuse considerable distances in tumors to reach their target ce ll population. This study uses a new three-dimensional tissue culture model, in which cells are grown as multicellular layers (MCL), to inve stigate whether metabolic consumption of TPZ is sufficiently rapid to compromise its extravascular diffusion in tumors. Methods and Material s: V79-171b and MGH-U1 cells were grown as MCL to thicknesses of appro ximately 120 and 360 mu m respectively. The extent of hypoxia in MCL, as assessed by EF5 binding, was modulated by altering gas-phase O-2 co ntent, and flux of TPZ through MCL was investigated by high-performanc e liquid chromatography (HPLC). Data were fitted to a diffusion-reacti on mathematical model to determine the diffusion coefficient of TPZ in the MCL (D-M) and the rate of its metabolic consumption under anoxia. These parameters were used to simulate TPZ transport in tumors. Resul ts: The flux of TPZ through well-oxygenated MCL (equilibrated with 95% O-2) was well fitted as Fickian diffusion without reaction, with a D- M of 7.4 x 10(-7) cm(2) s(-1) (12-fold lower than in culture medium) f or V79 and 1.3 x 10(-6) cm(2) s(-1) for MGH-U1 MCL. Flux of TPZ was su ppressed under anoxia, and fitting the data required inclusion of a re action term with a rate constant for metabolic consumption of TPZ of 0 .52 min(-1) for V79 and 0.31 min(-1) for MGH-U1 MCL. These transport p arameters would translate into a 43% or 30% decrease respectively in T PZ exposure, as a result of drug metabolism, in the center of a slab o f anoxic tissue 100 mu m in thickness. Conclusions: MCL cultures provi de an in vitro model for investigating the interaction between metabol ic consumption and diffusion of bioreductive drugs. If rates of diffus ion and metabolism similar to those measured in V79 and MGH-U1 MCL app ly in tumors, then cells in large confluent regions of hypoxia would b e partially protected by failure of TPZ penetration. Simulation of ext ravascular transport of TPZ-like bioreductive drugs demonstrates that the optimum metabolic rate constant is determined by two competing req uirements: it should be high enough to ensure potent cytotoxicity unde r hypoxia, yet low enough that penetration is not severely compromised . (C) 1998 Elsevier Science Inc.